Effect of induced thyroid dysfunction upon uterine responsiveness in strips from pregnant and nonpregnant rats

The influence of thyroid dysfunction on rat myometrial responsiveness to oxytocin, acetylcholine, CaCl2 and BaCl2 was assessed in uterine strips from pregnant and nonpregnant rats. In preparations obtained from hypothyroid nonpregnant rats, oxytocin and acetylcholine concentration-response curves were significantly displaced to the left, whereas myometrial strips from hyperthyroid nonpregnant rats were only supersensitive to acetylcholine. In relation to the pregnant state, T3 treatment caused an increased myometrial sensitivity to both oxytocin and acetylcholine. In this condition, concentration-response curves for oxytocin did not differ from nonpregnant control animals, whereas acetylcholine was about 300-fold more potent at the ED50 levels compared with those in nonpregnant animals. Either in pregnant or nonpregnant state, maximal responses and ED50 to BaCl2 did not differ. In addition, thyroid dysfunction did not modify the pattern of CaCl2-induced contractions in isolated myometrial strips from pregnant rats. This data extended to the myometrium, previous evidence in the literature indicating that thyroid dysfunction may affect uterine responsiveness to agonists.